On Great Plains, Juniper Invasion Signals Prairies in Distress

TULSA, Oklahoma—When the first Europeans made their way to the Great Plains in the center of the North American continent, they found vast prairies extending to the horizon, with the waves of grass hardly broken by trees. But a visitor to those same prairies today—from Texas up to South Dakota—is likely to find an army of shrubby junipers overtaking the grass.

March of the junipers. The trees’ aggressive spread “has emerged as a dominant threat to some of the most threatened ecosystems of North America,” says one researcher.
[CREDIT: Jay Pruett/Oklahoma Chapter of The Nature Conservancy]

Far more than an aesthetic change, the shifting ecology places both wildlife and humans at risk: The trees crowd into grazing land and consume water that might otherwise nourish the prairie or provide supplies for local towns and cities. They discharge clouds of highly allergenic pollen and harbor mosquitoes that carry West Nile virus. And they usurp habitat crucial to species such as the lesser prairie chicken and the prairie mole cricket.

“Juniper invasion has emerged as a dominant threat to some of the most threatened ecosystems of North America,” says Samuel Fuhlendorf, a professor in the Natural Resource Ecology and Management program at Oklahoma State University (OSU).

At the recent meeting here of the AAAS Southwestern and Rocky Mountain Division, Fuhlendorf and others detailed how human activity over the past 150 years has upset a natural balance that endured on the prairies for millennia. The question now—for farmers and ranchers, hunters, conservationists, consumers, policy-makers, and scientists—is whether humans can help restore the balance and the prairie’s health.

The 86th Annual Meeting of the Southwestern and Rocky Mountain Division convened 31 March to 4 April at the University of Tulsa (TU) to explore regional issues with global resonance—cybersecurity, new digital tools for archaeology, research ethics, and the relationship between science and religion, among others. The meeting was held in conjunction with the 15th Annual TU Research Colloquium and the 10th Annual University of Oklahoma-Tulsa Research Day.

Symposia on the juniper invasion and Oklahoma’s endangered species explored the health of the Great Plains, but the lessons apply to grasslands worldwide.

Eastern redcedar (Juniperus virginiana) and mountain cedar (Juniperus ashei) are native to the region, but for millions of years a natural cycle of wildfires held them in check.

Then, in the 19th century, European pioneers brought intensive cattle-grazing. The sodbusters soon followed, turning up millions of acres of native grass to plant wheat. After the disastrous Dust Bowl of the 1930s, thousands of Eastern redcedar were planted as windbreaks to hold the soil in place. Wildfires were suppressed as a threat to human communities.

Not until it was too late did people realize that the disrupted soil and lack of fires create favorable conditions for the juniper.

In 2002, the U.S. Department of Agriculture estimated that 8 million acres in Oklahoma alone were “infested” with 50-plus junipers per acre. The conquest was growing by 762 acres a day, doubling every 18 years. New birds follow the trees into the prairie; they eat the juniper berries and expel the seeds, aiding the invasion with each meal. Rising levels of atmospheric carbon dioxide, associated with climate change, may also be fueling the advance, researchers told the AAAS division audience.

Researchers see the lesser prairie chicken as symptomatic of the plains’ stressed condition. Agriculture, overgrazing, and energy development have helped reduce its population by 90% since 1900. Fuhlendorf says that junipers and wind turbines pose similar threats: The bird “tolerates basically no vertical structures,” he said, because it perceives them as haven for predators.

The impact of juniper proliferation is felt even in urban centers. Estelle Levetin, a TU aerobiologist, reported that eastern redcedars around Tulsa appear to be driving “a significant increase” in airborne juniper pollen. Early results in a study covering 25 years suggest a 110% increase in the city’s springtime pollen, she said.

Other research indicates that the junipers disrupt the prairie water cycle. Mature trees may consume up to 35 liters of water a day. Don Turton, a forest hydrologist at OSU, said that preliminary research results attribute a 20% average net loss of water to redcedar canopies. That leaves soil drier and reduces the flow of surface streams. In heavily infested watershed areas along the Canadian River, that may reduce water supplies for Oklahoma City.

In an arid state like Oklahoma, water is a rallying point for diverse interests, from ranchers and farmers to hunters, anglers, and environmentalists. Over the past decade, campaigns to control the junipers have intensified in many parts of the Great Plains. Some are working to conserve blocks of native grassland and reintroducing herds of buffalo. Others have established cooperatives to manage controlled burns, using new science to help bring an ancient balance back to the prairie.